1. Field of the Invention
The present invention concerns a control system for restrictively controlling an electromagnetically controlled differential.
2. Description of Related Art
Differentials drive both axles at same time but allow them to turn at different speeds when negotiating turns. Such a differential is typically comprised of differential pinions and cylindrical racks which are meshed with the differential pinions so as to hold them. Alteratively, a sun gear and planetary gears are often used as a differential. In order to restrict differential action of these differentials, mechanical clutches are internally provided. Hydraulically controlled types of mechanical clutches have conventionally been used. However, because of easy control, a recent tendency is to use what is called "electromagnetically controlled differentials" (which are hereafter referred to as EMC differentials) in which electromagnetically controlled clutches are installed in order to restrictively control differential action.
Reference is made to FIG. 8 for the purpose of providing a brief background of a general EMC differential, described in, for instance, Japanese Unexamined Patent Publication No. 63-192620, which will enhance an understanding of operation of the restrictive control system for the differential action of the present invention. An EMC differential, which is generally indicated by a numeral 1, is basically comprised of a planetary gear mechanism including a differential case 11, a planetary carrier 12 and a sun gear 13. The differential case 11 connects the engine output transmitted from a drive shaft (not shown) to the EMC differential 1. The drive shaft is an engine crankshaft if the EMC differential is a center differential; a front propeller shaft if it is a front differential; and a rear propeller shaft if it is a rear differential. The planetary carrier 12 connects the rotation of the drive shaft transmitted from the differential case 11 to a first driven shaft (not shown), which driven shaft is the front propeller shaft if the EMC differential is the center differential; one of the front axles if it is the front differential; and one of the rear axles if it is the rear differential. The sun gear 13 connects the rotation of the drive shaft transmitted from the differential case 1 to a second driven shaft (not shown), which driven shaft is the rear propeller shaft if the EMC differential is the center differential; another front axle if it is the front differential; and another rear axle if it is the rear differential.
The EMC differential 1 has two multiple disc clutches, such as a pilot clutch 14a and a main clutch 14b, each of which is actuated and deactuated by means of electric magnet 15. When the electric magnet 15 is energized, the planetary carrier 12 and the sun gear 13 are locked up together to restrict a differential action between the first and second driven shafts. While the electric magnet 15 is deenergized, the planetary carrier 12 and the sun gear 13 are free in operation from each other to allow a differential action between the first and second driven shafts. On the other hand, when the electromagnet 15 is energized, it attracts a metal disk 17, depressing clutch disks of the pilot clutch 14a so as to lock the pilot clutch 14a. The attraction of the metal disk 17 forces the differential case 11 to move axially toward the right as viewed in FIG. 8, so that the main clutch 14b is locked by the differential case 11 as well. As a result, the planetary carrier 12 and the sun gear 13 are mechanically coupled together with the differential case 11 through a pressure ring 16 and, accordingly, turn together so as to restrict a differential action between the first and second driven shafts, thereby creating the locked state of the differential 1.
Typically, four wheel drive vehicles are provided with EMC differentials of this type such as front, center and rear EMC differentials 1f, 1c and 1r, as schematically shown in FIG. 7. The center EMC differential 1c allows a differential action between front and rear wheels W1 and W2. The front and rear EMC differentials 1f and 1r allow differential actions between the front wheels W1 (W11 and W12) and between the rear wheels W2 (W21 and W22), respectively.
In the operation of such a four wheel drive vehicle provided with three EMC differentials, as described in the above mentioned publication, the transmission of torque to the front and rear axles is controlled by a function of transmissive torque of the clutch in such a manner that a bias or torque difference is provided between front and rear output shafts of a center differential.
However, in the conventional EMC differential control, if the engine stops while the EMC differentials are restricted in differential action or locked, each of the EMC differentials is cut off from the supply of current simultaneously with the stop of the engine, so that an abrupt unlocking of the EMC differential occurs. This abrupt unlocking allows the output shaft to have residual torque. Accordingly, at the time the EMC differential becomes free from restriction, i.e. unlocked, it generates shock, accompanied by an extraordinary clicking sound, due to which the vehicle vibrates.